The present invention relates generally to high-speed data communication systems, and specifically to Asymmetric Digital Subscriber Line (ADSL) systems.
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards have been defined, known generically as xe2x80x9cxDSL,xe2x80x9d wherein the various standards have different data rates and other associated features but share common principles of operation.
The present patent application is concerned mainly with Asymmetric DSL (ADSL) service, which allows data to be conveyed downstream to the subscriber at a rate of about 8 Mbit/s, and to be returned upstream from the subscriber at about 640 kbit/s. ADSL is based on a discrete multi-tone (DMT) transmission system, in which data are encoded using 256 different xe2x80x9ctones,xe2x80x9d each corresponding to a frequency band 4.3125 kHz wide. Recommendation G.992.1 (ex-G.dmt) of the International Telecommunication Union (ITU), which is incorporated herein by reference, specifies the physical layer characteristics of the ADSL interface to the subscriber line, including the allocation of the tones in the frequency spectrum to upstream and downstream service.
Annex A of G.992.1 specifies requirements for ADSL operating on a telephone line in the frequency band above that used for xe2x80x9cplain old telephone servicexe2x80x9d (POTS). Upstream ADSL service is allocated tones 6 through 31 (25.875 to 138 kHz). Downstream service may use all of tones 6 through 255 (25.875 up to 1104 kHz), as long as echo cancellation is applied in the upstream service range. Alternatively, downstream service may be limited to tones 32 through 255 if necessary to reduce near-end crosstalk (commonly referred to as xe2x80x9cNEXTxe2x80x9d) with the tones of the upstream signals. The range below about 25 kHz is left for POTS audio signals.
FIG. 1 is a schematic diagram showing allocation of frequencies when ADSL and ISDN (Integrated Service Digital Network) services are provided over a common subscriber loop, as specified in Annex B of G.992.1. The diagram schematically illustrates a set of spectral masks 20 given in the standard. The shapes of the masks are simplified here for clarity of illustration, the horizontal axis is not drawn to scale, and the vertical scale of power spectral density (PSD) is arbitrary, As shown in the figure, the range below 138 kHz (corresponding to tones 0-31) is unavailable to ADSL, as it must be left clear for ISDN transmission in a lower band 22. Tones 32 through 63 are allocated to ADSL upstream transmission in a middle band 24, while tones 32 through 255 are allocated to ADSL downstream transmission in an upstream band 26. In practical implementations, however, it is frequently necessary to limit the downstream transmission to an upper band 25, roughly covering the range of tones 64-255, in order to reduce NEXT.
ADSL service providers must be concerned not only with crosstalk between upstream and downstream transmissions on a single subscriber loop, but also with crosstalk between different loops in the same central office. This crosstalk can be particularly severe in the range of tones 32 through 63, which is allocated to downstream transmission in Annex A and to upstream transmission in Annex B. For this reason, service provider""s may find it necessary to use the Annex B ADSL spectrum not only when ADSL is provided over an ISDN line, but also for ADSL over POTS. This solution wastes valuable bandwidth and can severely degrade the ADSL downstream performance.
A range of variations on the standard ADSL spectral profiles have been proposed, in order to increase the upstream and/or downstream data rates while reducing crosstalk interference. For example, U.S. Pat. No. 5,519,731, whose disclosure is incorporated herein by reference, describes a DMT transmission scheme for use in ADSL, which is intended to mitigate T1 crosstalk noise in the ADSL signals. The scheme uses 512 subchannels, rather than the conventional 256 tones. Some of the added subchannels may be made available for Upstream communications, so as to increase the upstream data rate.
ITU Temporary Document NG-079r1, which is incorporated herein by reference, proposes an optimal asymmetric power spectral density mask for all-digital services, particularly for ADSL. In this proposal, the band of tones 0-6, which is set aside for POTS in the above-mentioned Annex A, is shared between upstream and downstream ADSL transmissions. In other words, both upstream and downstream masks are allowed to extend down to tone 0 in order to increase their data rate. Because NEXT typically increases strongly with frequency, however, the tones in the upper range of the upstream band, between about 90 and 138 kHz (roughly tones 21-32), are masked out of the downstream band.
It is an object of some aspects of the present invention to provide methods and apparatus for reducing crosstalk in ADSL transmissions.
It is a further object of some aspects of the present invention to provide methods and apparatus that enable improved exploitation of the spectrum available for ADSL, particularly in the presence of other network services, such as ISDN.
In preferred embodiments of the present invention, an improved spectral utilization scheme is introduced for use in ADSL service on a group of subscriber loops, at least some of which are also used for an existing digital network service, such as ISDN. Lower, middle and upper frequency bands are defined for transmission over the subscriber loops. The existing digital network service is provided on the lower band, and the middle and upper bands are used respectively for upstream arid downstream ADSL transmission. This scheme accords with G.992.1 Annex B. For those subscriber loops that do not receive ISDN service, however, the lower band, at least down to the POTS range, is exploited to provide additional transmission bandwidth for both upstream and downstream ADSL transmissions.
Preferred embodiments of the present invention offer a number of advantages over ADSL spectral utilization schemes known in the art. The use of both the lower and middle bands for upstream transmission by non-ISDN subscribers increases the available upstream bandwidth. On the other hand, because there is no downstream transmission in the middle band, crosstalk in this band is substantially eliminated, including both NEXT on individual subscriber loops and inter-subscriber crosstalk between the non-ISDN and Annex B subscriber loops that are served by the same central office. Non-ISDN subscribers thus receive ADSL service with substantially greater bandwidth than can generally be offered in a mixed Annex B and non-ISDN environment that uses methods and apparatus known in the art.
In some preferred embodiments of the present invention, a modem is configurable so as to provide either Annex B service, combining ADSL and ISDN transmission, or ADSL service (with or without POTS) using the novel spectral utilization scheme described hereinabove. This type or modem may then be used for all of the subscribers served by the central office. The choice of Annex B or non-ISDN service is implemented simply by switching the lower frequency band in or out of ADSL use, using either hardware or software switching. Alternatively, different modems may be used for different subscribers, depending on their choice of services.
Although preferred embodiments are described herein with reference to ADSL and ISDN services and certain specific spectral bands used for these services, it will be appreciated that the principles of the present invention may similarly be applied to other types of digital transmissions, and particular to transmissions in accordance with other standards in the xDSL family.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for providing Digital Subscriber Line (DSL) service to a group of subscribers over respective subscriber loops in the presence of another, existing digital network service provided on at least some of the subscriber loops, the method including:
assigning lower, middle and upper frequency bands for transmission over the subscriber loops, such that the existing digital network service is provided on the lower band;
transmitting DSL signals downstream on the upper band and receiving DSL signals upstream on the middle band, generally without transmitting or receiving DSL signals on the lower band, over those subscriber loops on which the existing digital network service is provided; and
transmitting DSL signals downstream on the lower and upper bands and receiving DSL signals upstream on the lower and middle bands over those subscriber loops on which the existing digital network service is not provided.
Preferably, the existing digital network service includes an Integrated Service Digital Network (ISDN) service, and the DSL service includes Asymmetric DSL (ADSL) service. Most preferably, transmitting the DSL signals over those subscriber loops on which the existing digital network service is provided includes transmitting ADSL signals substantially in accordance with Annex B of standard G.992.1 of the International Telecommunications Union. Alternatively or additionally, the method includes transmitting plain old telephone service (POTS) signals over those subscriber loops on which the existing digital network service is not provided.
Preferably, the DSL service includes Asymmetric DSL (ADSL) service, and wherein the lower band includes approximately the range of ADSL tones 6 through 31, and the middle band includes approximately the range of ADSL tones 32 through 63.
In a preferred embodiment, transmitting the DSL signals over those subscriber loops on which the existing digital network service is not provided includes transmitting the signals via a filter that is configurable for transmitting the DSL signals on the lower band, and transmitting the DSL signals over those subscriber loops on which the existing digital net-work service is provided includes switching the configurable filter so as not to transmit the DSL signals over the lower band.
Preferably, receiving the DSL signals upstream on the lower and middle bands over those subscriber loops on which the existing digital network service is not provided includes canceling echoes due to the DSL signals transmitted downstream in the lower band. Most preferably, receiving the DSL signals upstream on the lower and middle bands includes substantially eliminating reception of upstream signals in a range of tones near an upper edge of the middle band, and wherein transmitting the DSL signals downstream includes substantially eliminating transmission of downstream signals in a range of tones near a lower edge of the upper band, whereby only echoes in the lower band need be canceled.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for providing Asymmetric Digital Subscriber Line (ADSL) service to a subscriber over a subscriber loop, the method including:
transmitting ADSL signals downstream over the subscriber loop in a lower band consisting approximately of the range of ADSL tones 6 through 31 and in an upper band consisting approximately of the range of ADSL tones 64 and above, but substantially not in a middle band between the lower and upper bands; and
receiving ADSL signals upstream from the subscriber loop in the lower band and in the middle band.
Preferably, receiving the ADSL signals Upstream includes receiving and processing signals only in the lower band when the subscriber is not equipped to transmit signals in the middle band.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for providing Asymmetric Digital Subscriber Line (ADSL) service to a subscriber over a subscriber loop, the method including:
transmitting ADSL signals downstream over the subscriber loop in a lower band consisting approximately of the range of ADSL tones 6 through 31 and in an upper band consisting approximately of the range of ADSL tones 64 and above, but substantially not in a middle band between the lower and upper bands; and
receiving ADSL signals upstream from the subscriber loop in the lower band.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for providing Digital Subscriber Line (DSL) service to a subscriber over a subscriber loop, the modem including:
downstream processing circuitry, configured to generate DSL signals to be transmitted over the subscriber loop in a lower frequency band, having A low-frequency cut-off above a plain old telephone service (POTS) band, and in an upper frequency band, but substantially not in a middle frequency band between the upper and lower bands; and
upstream processing circuitry, configured to receive DSL signals upstream from the subscriber loop in the lower band and in the middle band.
In a, preferred embodiment, the downstream and upstream processing circuitry is configurable so as to limit the DSL signals to the middle and upper bands, whereby the lower band is made available for another digital network service.
Preferably, the apparatus includes a line coupling circuit, which is adapted to couple both the downstream and upstream DSL processing circuitry and signals associated with the other digital Network service to the subscriber loop. In a preferred embodiment, the apparatus includes a high-pass filter, operative to implement the low-frequency cut-off of the lower band in the DSL signals, at approximately an upper edge of the plain old telephone service (POTS) band, and a low-pass filter, operative to implement a high-frequency cut-off in the signals associated with the other digital network service, at approximately a high-frequency limit of the other digital network service, substantially above the upper edge of the POTS band. Preferably, the upstream processing circuitry includes a configurable filter, which cuts off signals in the lower band when the lower band is used for the other digital network service.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, Digital Subscriber Line (DSL) subscriber equipment, adapted to be coupled to a subscriber loop over which another existing digital network service may also be provided on a lower frequency band, the equipment including:
a first DSL modem, which is configured to receive DSL signals transmitted upstream on a middle frequency band above the lower frequency band and to transmit DSL signals downstream on an upper frequency band above the middle frequency band, such that the lower frequency band is substantially free of the DSL signals, over a first one of the subscriber loops, on which the existing digital network service is provided; and
a second DSL modem, which is configured to receive DSL signals transmitted upstream on the lower and middle bands and to transmit DSL signals downstream on the lower and upper bands over a second one of the subscriber loops, on which the existing digital network service is not provided.
Preferably, the first and second DSL modems include circuitry that is configurable so as to enable selection of either the DSL signals or the existing digital network service for transmission on the lower band.
Further preferably, the system includes subscriber equipment, coupled to at least one of the first and second subscriber loops so as to communicate with the respective first or second DSL modem, wherein the subscriber equipment includes circuitry that is configurable so as to enable selection of either the DSL signals or the existing digital network service for reception on the lower band.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, Digital Subscriber Line (DSL) subscriber equipment, adapted to be coupled to a subscriber loop over which another existing digital network service may also provided on a lower frequency band, the equipment including:
a DSL modem; and
a splitter, which is coupled to convey signals having frequencies above the lower frequency band between the subscriber loop and the DSL modem, and which is switchable so as to couple signals on the lower frequency band either to equipment associated with the existing digital network service or to the DSL modem.
There is additionally provided, in a Digital Subscriber Line (DSL) service environment, wherein DSL service is provided to a group of subscribers over respective subscriber loops in the presence of another, existing digital network service provided on a lower frequency band on at least some of the subscriber loops, a method for accessing the DSL service on the subscriber loops on which the other, existing digital network service is not provided, the method including:
transmitting upstream DSL signals on the lower band and on a middle band above the lower band; and
receiving downstream DSL signals on the lower band and on an upper band above the middle band, generally without receiving downstream signals on the middle band.